I am not a scholar but I do not need one to tell me the meanings of this verse from Quran,
4 - 137. Verily, those who believe, then disbelieve, then believe (again), and (again) disbelieve, and go on increasing in disbelief; Allah will not forgive them, nor guide them on the (Right) Way.
Because common sense tells me that those who believe, then disbelieve, then believe and then disbelieve........................means that they should not be killed after "first or even second disbelief".